Lost an important note (while adding storage)

Help!

I use my iphone 6 for work and especially the Notes. Tonight, as I added something to one VERY important Note, I got a pop up saying I had run out of storage. I bought some more storage and returned to the Note only to find it had disappeared. I also checked my iPad and it had disappeared from there too. As far as I can tell all my other Notes are still there. What happened? Did my phone just not have enough storage for the one extra word I added so it deleted the whole Note? I'm majorly panicking right now. Can I reset my phone to last night's back up? Or how do I get the Note back?

Thank you for any help you can offer. (I'll also need the idiot's guide!! )